One of my all-time favorite mods is the Lycanite's Mobs Mod. It's got some Doom-like mobs as well as some other fantasy creatures. It's a fun mod to play, especially with the new Nether boss, Rahovart. There isn't much new stuff as far as plants and blocks go, but it's still really interesting and you can have a lot of fun playing it.

Here's the link to their website: http://lycanitesmobs.com/